Research On Signal Processing System Of Frequency-Modulated Continuous-Wave Interferometric Fiber-Optic Displacement Sensor

Optical frequency-modulated continuous-wave(FMCW)interference,a new optical measurement technology derived from Radar,can be used to measure strain,temperature,pressure,displacement,rotation velocity by calibrating the fractional phase,distinguishing the phase-shift direction,and counting the number of full periods for interferometric signal.The theory of optical Frequency-Modulated continuous-wave interferometers with high accuracy and large dynamic range was discussed in details.Based on the structure of the Fabry-Perot interferometer,the interferometric optical path is constructed with optical fiber devices.A signal processing system for fiber-optic FMCW displacement sensors is designed and implemented with high precision and large dynamic range.Using the sawtooth linear frequency modulation as an example,operational principle of the fiber-optic FMCW displacement sensor was analyzed,and an reciprocal fiber-optic path was introduced.Through the linear modulation of the driving current for a DFB semiconductor laser;a driving circuit for the linear frequency modulation of the DFB semiconductor laser was accomplished.Based on the STM32F405VGT6 processor,a signal processing system was developed,including the circuit schematic design of a photoelectric conversion circuit,a STM32 minimal system,a AD acquisition system,a serial communication system,a LCD display and keyboard input circuit.A reliable electronic hardware platform for fiber-optic FMCW displacement sensors was built by platemaking and debugging the PCB circuit.Aiming at solving problems of low signal-to-noise ratio,uneqaul-amplitude and uneqaul-period in each modulation cycle for the beat signal of fiber-optic FMCW displacement sensors,a peak-seeking phase detection method with high-precision was proposed in this paper.The phase detection was realized by peak detection method,in which a raised cosine function as a template function of a cross-correlation method was carried out to improve the signal-to-noise ratio,and a golden section method was adopted to reduce the computational complexity of the cross-correlation operation.On the hardware platform of the fiber-optic FMCW displacement sensor,software design of the signal processing system and transplantation of phase demodulation algorithm were completed to realize high-precision displacement measurement.The measurement results can be sent to a host computer through a serial port for accuracy analysis.The principle prototype of frequency-modulated continuous-wave fiber-optic displacement sensor is built to test the function of signal processing system and linear frequency-modulated DFB semiconductor laser.The actual displacement measurement experiment is carried out by using electric displacement table.The results of experiment show that the frequency-modulated continuous-wave fiber-optic displacement sensor can complete the displacement measurement range varying from 10nm to 400mm.In the 300-400 mm measurement experiment,the repeatability error of displacement measurement is less than 3.5 nm and the standard error is 4.5 nm.